# Why is there a resistor bridge before the amplifier?

I really fail to understand what the resistor bridge before the amplifier does. Can I just connect my inputs to the - and + ports?

This was taken from the datasheet for INA125, and they use this configuration in several of the applications, and I really can't understand why...

• In that circuit, the resistor bridge is the sensor which provides the input signal to the amplifier. The +3V and GND connections power the sensor bridge. Jan 12, 2017 at 17:22

The bridge is there because the figure is meant to illustrate (like the caption says) a "bridge amplifier". This means its purpose is to amplify the output signal of a bridge.

If your signal source is not a bridge, then this circuit doesn't apply to you. How you should connect your inputs depends on what kind of source you have.

It's a Wheatstone bridge which is used in sensors to measure an unknown resistance. Some sensors that use Wheatstone bridges are magnetometers, strain gauges, thermistor circuits to name a few.

You can find the resistance with this equation (if there is no instrumentation amplifier the Gain term is 1) $$V_G = Gain*(V_+ - V_-) = Gain*(\frac{R_2}{R_1+R_2}-\frac{R_x}{R_x+R_4})$$

The nice thing about the INA125 is it has a reference voltage for the bridge. Considering the gain of the reference and gain of the instrumentation amplifier which is set by $R_g$

$$V_O = V_{ref} + Gain*(V_+ - V_-) = V_{ref} + (4+\frac{60kΩ}{R_g})*(V_+ - V_-)$$

If you don't use a Wheatstone bridge as your sensor, an instrumentation amplifier is like a multi meter, it measures the difference in voltage. So you can use the $V+$ and $V-$ terminals to get the difference in voltage between them.

$$V_O = V_{ref} + Gain*(V_+ - V_-)$$

Setting R_G to 60kΩ will give you a gain of 1

Keep in mind that unlike a multimeter, instrumentation amplifiers can only measure the range inside of their power rails (ground and Vcc).

• Strictly, a Wheatstone bridge is used to measure an unknown resistance. Bridges, generally, are not balanced, but give an output voltage dependent on small variation of one or more of the resistances or impedances. For example, a strain gauge bridge.
– Chu
Jan 12, 2017 at 18:10